1. Field of the Invention
The present invention relates to a plastic lens, molded from a suitable transparent thermoplastic resin, which may be used, for example, in an optical reading system of a laser-disk reader for reading optical signals from a laser-disk with a laser beam projected through the optical reading system.
2. Description of the Related Art
Conventionally, in an optical reading system of a laser-disk reader, a molded small sized plastic lens, which is formed of a suitable transparent thermoplastic resin, is used as an objective lens. This plastic lens is assembled in a lens holder incorporated in the optical reading system of the laser-disk reader. In assembling the plastic lens in the lens holder, the plastic lens must be correctly positioned in place with respect to the lens holder, such that optical signals can be read from a laser-disk by the plastic lens.
To correctly position the plastic lens with respect to the lens holder, the plastic lens has two positioning surfaces, which are arranged so as to abut two corresponding positioning surfaces provided on the lens holder. Namely, the assembly of the plastic lens in the lens holder is carried out such that the respective positioning surfaces of the plastic lens abut the corresponding positioning surfaces of the lens holder, resulting in the plastic lens being properly positioned with respect to the lens holder.
Specifically, the plastic lens includes a lens body, a small-diameter collar portion integrally formed around the lens body and a large-diameter collar portion integrally formed around the lens body. An annular shoulder surface is defined between the small-diameter collar portion and the large-diameter collar portion, and serves as a first positioning surface, which extends perpendicularly with respect to an optical axis of the lens body. A peripheral cylindrical surface of the small-diameter collar, which extends perpendicularly with respect to the first positioning surface, has a central longitudinal axis coinciding with the optical axis of the lens body, and serves as a second positioning surface.
The lens holder has a bore formed therein, and a flat annular surface area surrounding a peripheral circular edge of an opening of the bore, which extends perpendicularly with respect to a central longitudinal axis of the bore. The flat annular surface area serves as a first positioning surface, which operates in conjunction with the first positioning surface of the plastic lens. A peripheral cylindrical surface of the bore, which operates in conjunction with the second positioning surface of the plastic lens, extends perpendicularly with respect to the first positioning surface, and serves as a second positioning surface.
The second positioning surface of the lens holder, having an inner diameter substantially equal to an outer diameter of the small-diameter collar portion of the plastic lens, allows assembly of the plastic lens in the lens holder to be carried out by fitting the small-diameter collar portion of the plastic lens into the bore. Namely, when the small-diameter collar portion of the plastic lens is fitted into the bore, the plastic lens can be correctly positioned in place with respect to the lens holder, so that the optical axis of the plastic lens and the central longitudinal axis of the lens holder coincide with each other. Nevertheless, the completely assembled conventional plastic lens is bulky, due to the existence of the first and second collar portions integrally formed around the lens body.